Natural gas liquefaction process

ABSTRACT

Disclosed herein is a natural gas liquefaction process of pre-cooling natural gas using a closed loop pre-cooling cycle and liquefying the pre-cooled natural gas using a closed loop liquefying cycle, wherein the closed loop pre-cooling cycle includes first and second pre-cooling cycles in parallel for pre-cooling supplied natural gases together in the same first heat exchange region through the respective pure refrigerants, and the closed loop liquefying cycle includes at least one liquefying cycle for liquefying the pre-cooled natural gas through a mixed refrigerant, the first and second pre-cooling cycles being a closed circuit cooling cycle.

TECHNICAL FIELD

The present invention relates to a natural gas liquefaction process, andmore particularly, to a natural gas liquefaction process capable ofhaving excellent efficiency, decreasing the number of equipments,simplifying the structure of the liquefaction system, and easilyoperating the liquefaction system by configuring a pre-cooling cycle sothat the advantages of a pure refrigerant cycle and a mixed refrigerantcycle may be taken at the same time.

BACKGROUND ART

A thermodynamic process of liquefying natural gas to produce liquefiednatural gas (LNG) has been developed since the 1970s in order to satisfydemands for higher efficiency, larger capacity, simpler equipment, andso on. In order to satisfy these demands, various attempts to liquefynatural gas using different refrigerants or different cycles have beencontinuously conducted up to now. However, the number of liquefactionprocesses that are practically under current operation is very small.

One of the liquefaction processes that are being operated and have beenmost widely spread is a ‘propane pre-cooled mixed refrigerant process(or a C3/MR process)’. A basic structure of the C3/MR process is shownin FIG. 9. For reference, in FIG. 9, and the like, ‘C3’ indicates apropane refrigerant cycle, and ‘MR’ indicates a mixed refrigerant cycle.Further, in FIG. 9, and the like, ‘C’ indicates a compressor, ‘AC’indicates an after-cooler, ‘V’ indicates a valve, and ‘HX’ indicates aheat exchanger.

As shown in FIG. 9, the feed gas is pre-cooled down to approximately 240K by a multi-stage of propane (C3) cooling cycle. The pre-cooled feedgas is condensed and sub-cooled down to approximately 113 K by a mixedrefrigerant cycle, that is, by heat exchange with mixed refrigerant (MR)in a heat exchanger. Also in this C3/MR process, general features of apure refrigerant cycle and a mixed refrigerant cycle appear as they are.In pure refrigerant cycles, a structure is simple and an operation iseasy, but a large number of refrigeration stages are required. On thecontrary, in mixed refrigerant cycles, a structure is complicated and anoperation is difficult, but high efficiency may be obtained only with asmall number of components. These features of the respective cyclesappear as they are also in the C3/MR process.

More specifically, in the mixed refrigerant cycle of the C3/MR processof liquefying (and sub-cooling) the pre-cooled feed gas, the mixedrefrigerant composed of nitrogen, methane, ethane, and propane, isgenerally used. In the mixed refrigerant cycle of the C3/MR process,high efficiency may be obtained using only a small number of equipmentsby appropriately selected compositions of these components,appropriately separated mixed refrigerant into a gas-phase refrigerantportion and a liquid-phase refrigerant portion according to a differencein boiling points among the respective components, and then liquefyingthe natural gas through the respective refrigerant portions. On theother hand, in the pure refrigerant cycle of the C3/MR process ofpre-cooling the feed gas, since a pure refrigerant such as propane isused, the structure is simple and the operation is easy, but three orfour pressure steps are required so that a large number of compressors,and the like, are required. As a result, in the case of the C3/MRprocess, the pre-cooling cycle may be considered as being focused onsimplicity (even though the number of equipments is many, the structureitself is simple), and the liquefying cycle may be considered as beingfocused on efficiency (even though the number of equipments is small,the structure itself is complicated and efficiency is excellent).

One of the other successful liquefaction processes that are beingoperated is a ‘dual mixed refrigerant process (or a DMR process)’. Thebasic structure of the DMR process is shown in FIG. 10. As shown in FIG.10, a liquefying (sub-cooling) cycle of the DMR is basically the same asthe liquefying (sub-cooling) cycle of the C3/MR process. However, in theDRM process, another mixed refrigerant cycle is used in order topre-cool the feed gas unlike the C3/MR process. In the pre-cooling cycleof the DMR process, a gas-liquid separator is not generally presentunlike the liquefying cycle in the DMR process. As a result, in the caseof the DMR process, both of the pre-cooling cycle and the liquefyingcycle may be considered as being focused on efficiency. However, it hasbeen known that the efficiency of the DMR process is slightly smallerthan that of the C3/MR process.

As described above, it has been known that the efficiency of the mixedrefrigerant cycle is higher than that of the pure refrigerant cycle.However, the structure itself of the mixed refrigerant cycle is morecomplicated than that of the pure refrigerant cycle. In addition, manyschemes of applying the mixed refrigerant cycle to a cooling cycle forliquefying (sub-cooling) the pre-cooled natural gas to increase theefficiency of the entire liquefaction process have been suggested.Therefore, many studies on a pre-cooling cycle having a simple structureand excellent efficiency have been demanded.

DISCLOSURE Technical Problem

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art while advantagesachieved by the prior art are maintained intact.

One objective to be achieved by the present invention is to provide anatural gas liquefaction process capable of having excellent efficiency,decreasing the number of equipments of a liquefaction system,simplifying a structure of the liquefaction system, and easily operatingthe liquefaction system by configuring a pre-cooling cycle so thatadvantages of both pure refrigerant cycle and mixed refrigerant cyclemay be taken.

Technical Solution

In one aspect of the present invention, there is provided a natural gasliquefaction process of pre-cooling natural gas using a closed looppre-cooling cycle and liquefying the pre-cooled natural gas using aclosed loop liquefying cycle, wherein the closed loop pre-cooling cycleincludes two pre-cooling cycles for pre-cooling supplied natural gasestogether in the same first heat exchange region through the respectivepure refrigerants, and the closed loop liquefying cycle includes atleast one liquefying cycle for liquefying the pre-cooled natural gasthrough a mixed refrigerant, the two pre-cooling cycles being a closedcircuit cooling cycle.

The pure refrigerant of the first pre-cooling cycle may be ethane (C2),and the pure refrigerant of the second pre-cooling cycle may be butane(C4). Each pre-cooling cycle may include a step of compressing the purerefrigerant, a step of cooling the compressed refrigerant, a step ofadditionally cooling the cooled refrigerant in the first heat exchangeregion, and a step of expanding the additionally cooled refrigerant.

The closed loop liquefying cycle may include a step of compressing themixed refrigerant, a step of cooling the compressed refrigerant, a stepof additionally cooling the cooled refrigerant in the first heatexchange region to partially condense the cooled refrigerant, a step ofseparating the partially condensed refrigerant into a liquid-phaserefrigerant portion and a gas-phase refrigerant portion according to adifference in boiling points, a step of primarily cooling the pre-coolednatural gas in a second heat exchange region using the liquid-phaserefrigerant portion, and a step of secondarily cooling the primarilycooled natural gas in a third heat-exchange region using the gas-phaserefrigerant portion.

The step of primarily cooling the pre-cooled natural gas may include afirst step of cooling the liquid-phase refrigerant portion through heatexchange in the second heat exchange region, a second step of expandingthe refrigerant portion cooled in the first step, and a third step ofheat-exchanging the refrigerant portion expanded in the second step andthe natural gas in with each other the second heat exchange region tocool the natural gas. The step of secondarily cooling the primarilycooled natural gas may include a cooling step of cooling the gas-phaserefrigerant portion through heat exchange in the second heat exchangeregion, a condensing step of condensing the refrigerant portion cooledin the cooling step through heat exchange in the third heat exchangeregion, an expanding step of expanding the refrigerant portion condensedin the condensing step, and a step of heat-exchanging the refrigerantportion expanded in the expanding step and the natural gas with eachother in the third heat exchange region to cool the natural gas.

Advantageous Effects

In the natural gas liquefaction process according to an exemplaryembodiment of the present invention, since the pre-cooling cyclespre-cool the natural gas only with a single pressure step, they may beconfigured of only a relatively small number of equipments.

In addition, in the natural gas liquefaction process according to anexemplary embodiment of the present invention, since the respectivepre-cooling cycles use the pure refrigerant, the structure itselfthereof is simple, and the operation is easy.

Furthermore, in the natural gas liquefaction process according to anexemplary embodiment of the present invention, since two pre-coolingcycles are disposed in parallel with each other to pre-cool the naturalgas in the same heat exchange region, the efficiency of the liquefactionprocess is excellent.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing the natural gas liquefaction processaccording to an exemplary embodiment of the present invention;

FIG. 2 is a graph showing the temperature profile in a pre-coolingregion of a C3/MR process according to the prior art;

FIG. 3 is a graph showing the temperature profile in a pre-coolingregion of a DMR process according to the prior art;

FIG. 4 is a graph showing the temperature profile in a pre-coolingregion of a liquefaction process according to the exemplary embodimentof the present invention;

FIG. 5 is a temperature-entropy diagram of ethane and butane cycles inthe liquefaction process according to the exemplary embodiment of thepresent invention;

FIGS. 6 to 8 are graphs showing exergy utilization and irreversibilityin a pre-cooling step of the C3/MR process according to the prior art,the DMR process according to the prior art, and the liquefaction processaccording to the exemplary embodiment of the present invention,respectively;

FIG. 9 is a flow chart showing the C3/MR process according to the priorart; and

FIG. 10 is a flow chart conceptually showing the DMR process accordingto the prior art.

BEST MODE

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to these exemplaryembodiments. For reference, the same reference numerals will be used todescribe substantially the same components. Under this rule, adescription may be provided while citing a content shown in otherdrawings. A content well-known to those skilled in the art to which thepresent invention pertains or a repeated content may be omitted.

FIG. 1 is a flow chart showing a natural gas liquefaction processaccording to an exemplary embodiment of the present invention. Theliquefaction process according to the present embodiment may be appliedto a natural gas liquefaction process of pre-cooling natural gas using aclosed loop pre-cooling cycle and liquefying the pre-cooled natural gasusing a closed loop liquefying cycle, as shown in FIG. 1. In addition,the liquefaction process according to the present embodiment may furtherinclude a cooling cycle of further cooling a mixed refrigerant orfurther cooling the natural gas.

Hereinafter, the liquefaction process according to the exemplaryembodiment of the present invention will be described with reference toFIG. 1. The liquefaction process according to the present embodimentbasically includes a closed loop cooling cycle of pre-cooling suppliednatural gas and a closed loop cooling cycle of liquefying (or liquefyingand sub-cooling) the pre-cooled natural gas. Since both of the coolingcycles according to the present embodiment are the closed loop cycles,the two respective cycles are independently performed as one closedcycle while having a compressing step, a condensing step, an expandingstep, and an evaporating step. In addition, the closed loop coolingcycle for pre-cooling the supplied natural gas, that is, the closed looppre-cooling cycle includes two separate cycles. These two pre-coolingcycles are also closed loop cooling cycles.

The liquefaction process according to the present embodiment will bedescribed in detail. As shown in FIG. 1, the supplied natural gas ispre-cooled by the first pre-cooling cycle and the second pre-coolingcycle in the first heat exchange region 110 (as described below, in theliquefaction process according to the present embodiment, the firstpre-cooling cycle is an ethane (C2) cycle and the second pre-coolingcycle is a butane (C4) cycle). That is, the supplied natural gas ispre-cooled by a pure refrigerant of the first pre-cooling cycle and apure refrigerant of the second pre-cooling cycle in the same heatexchange region 110. To this end, each of the first pre-cooling cycleand the second pre-cooling cycle has a compressing step, a condensingstep, an expanding step, and an evaporating step of a refrigerant.

In the first and second pre-cooling cycle as described above, the purerefrigerants are first introduced into compressors 151 and 161 throughconduits 201 and 401 to thereby be compressed. Then, the purerefrigerants are introduced into coolers 152 and 162 through conduits202 and 402 to thereby be cooled. The above-mentioned compressing andafter-cooling process may be performed in a multi-stage as shown inFIG. 1. That is, a plurality of compressors and after-coolers may beconnected in series with each other. In this case, the cooled purerefrigerants may be again introduced into compressors 153 and 163through conduits 203 and 403 to thereby be compressed, and the purerefrigerants compressed as described above may be again introduced intocoolers 154 and 164 through conduits 204 and 404 to thereby be cooled.When the compressor is configured as the multi-stage to compress thepure refrigerants in the multi-stage, required input power of thecompressor may be decreased.

In addition, the pure refrigerants compressed and cooled as describedabove may be introduced into the first heat exchange region 110 throughconduits 205 and 405 and heat-exchanged with a fed-back refrigerant tothereby be further cooled. The pure refrigerants may be condensedthrough the above-mentioned process. However, as described below,according to boiling points of the pure refrigerant, the purerefrigerants may also be condensed by cooling by the above-mentionedcooler. In this case, the pure refrigerants in the condensed state maybe introduced into the first heat exchange region 110 to thereby befurther cooled. The cooling of the refrigerants in the first heatexchange region 110 as described above may be performed by a refrigerantagain introduced into the heat exchange region 110 through conduits 207and 407. That is, the pure refrigerants cooled through the heat exchangein the first heat exchange region 110 as described above may beintroduced into expansion valves 155 and 165 through conduits 206 and406 to thereby be expanded and cooled and be then introduced again intothe first heat exchange region 110 through the conduits 207 and 407 tocool the natural gas and the refrigerant.

The above-mentioned processes are similarly performed in the firstpre-cooling cycle and the second pre-cooling cycle. That is, as shown inFIG. 1, the first pre-cooling cycle and the second pre-cooling cycle aredisposed in parallel with each other while forming a closed loop cycle,thereby complementarily pre-cooling the supplied natural gases in thefirst heat exchange region 110. As described below, the most importantfeature in the liquefaction process according to the present embodimentis to dispose two closed loop cooling cycles using the pure refrigerantin parallel with each other as described above to pre-cool the suppliednatural gases in the same heat exchange region.

In the present embodiment, ethane (C2) and butane (C4) are used as thepure refrigerants of the first and second pre-cooling cycles,respectively. In the C3/MR process described above, a pure refrigerantcomposed of propane (C3) is used in order to pre-cool the natural gas,and in the DMR process described above, a mixed refrigerant composed of45.5 mole % of ethane (C2), 4.9 mole % of propane (C3), and 49.6 mole %of butane (C4) is used in order to pre-cool the natural gas. That is, inthe C3/MR process described above, since a single pure refrigerant isused, a plurality of pressure levels are required, such that a largenumber of equipments are required, but a structure itself is simple andan operation of a pre-cooling cycle is easy. On the other hand, in theDMR process described above, since the mixed refrigerant is used, asmall number of equipments are required, but a structure itself iscomplicated and an operation of a pre-cooling cycle is also difficult.

In the liquefaction process according to the present embodiment, the twopure refrigerant cycles disposed in parallel with each other asdescribed above are used so that both of the advantages of two basicstructures as described above, that is, the advantages in the case ofusing the pure refrigerant for the pre-cooling and the case of using themixed refrigerant for the pre-cooling may be taken. In addition, the twopure refrigerant cycles are configured of an ethane cycle and a butanecycle, thereby making it possible to optimize the entire efficiency ofthe liquefaction process. The mixed refrigerant in the pre-cooling stepin the DMR process described above is composed of ethane, propane, andbutane components, wherein an amount of propane component included inthe mixed refrigerant is very small. In the liquefaction processaccording to the present embodiment, a pre-cooling effect due to theabove-mentioned propane component may also be considered as beingreplaced by a corresponding effect of the ethane cycle and the butanecycle.

The natural gas pre-cooled through the two pure refrigerant cycles asdescribed above is liquefied (or liquefied and sub-cooled) through themixed refrigerant cycle. More specifically, the mixed refrigerantpartially condensed through the heat exchange in the first heat exchangeregion 110 is introduced into a gas-liquid separator 171 through aconduit 301 to thereby be separated into a first refrigerant portion anda second refrigerant portion having a boiling point lower than that ofthe first refrigerant portion according to a difference in boilingpoints. That is, the partially condensed mixed refrigerant may beseparated into the first refrigerant portion separated as a liquid-phaserefrigerant portion due to a high boiling point and the secondrefrigerant portion separated as a gas-phase refrigerant portion due toa low boiling point.

The separated first refrigerant portion is introduced into a second heatexchange region 120 through a conduit 302 to thereby be cooled. Thecooling of the refrigerant portion described above may be performedthrough heat exchange with a refrigerant introduced into the second heatexchange region 120 through a conduit 304. The cooled refrigerantportion is introduced into an expansion valve 172 through a conduit 303to thereby be expanded. The expanded refrigerant portion may be mixedwith a second refrigerant portion to be described below and be thenintroduced again into the second heat exchange region 120 through theconduit 304 to cool other refrigerants and liquefy the natural gas.Then, after the mixed refrigerant is subjected to a series ofcompressing and after-cooling processes, it may be introduced into thefirst heat exchange region 110 to thereby be cooled together with thesupplied natural gas through the ethane cycle and the butane cycle.

In addition, the separated second refrigerant portion is introduced intothe second heat exchange region 120 through a conduit 306 to thereby becooled. The cooling of the refrigerant portion described above may beperformed through heat exchange with a refrigerant introduced into thesecond heat exchange region 120 through a conduit 304. The cooledrefrigerant portion is introduced into a third heat exchange region 130through a conduit 307 to thereby be condensed. The condensing of therefrigerant portion described above may be performed through heatexchange with a refrigerant introduced into the third heat exchangeregion 130 through a conduit 309. The condensed refrigerant portion isintroduced into an expansion valve 173 through a conduit 308 to therebybe expanded. The expanded refrigerant portion is again introduced intothe third heat exchange region 130 through the conduit 309 to condense arefrigerant introduced into the third heat exchange region 130 andliquefy or sub-cool the natural gas through the heat exchange. Forreference, the liquefied natural gas may be expanded by the expansionvalve 181 and be then introduced into a storing tank, or the like.

The refrigerant portion of which the heat exchange in the third heatexchange region 130 is finished may be mixed with the above-mentionedfirst refrigerant portion and be then introduced again into the secondheat exchange region 120. For reference, the above-mentioned three heatexchange regions 110, 120, and 130 may be provided together in a singleheat exchange unit as shown in FIG. 1 or be individually provided inthree heat exchange units. In addition, the heat exchange unit may be ageneral heat exchanger.

An effect of the liquefaction process according to the presentembodiment having the above-mentioned configuration will be describedwith reference to FIGS. 2 to 8. FIGS. 2 and 3 show temperaturedistributions in pre-cooling regions of the above-mentioned C3-MRprocess and DMR process, respectively. Since propane (C3) in the C3-MRprocess is a pure refrigerant and is subjected to several process steps,a temperature distribution has a stair shape as shown in FIG. 2. On theother hand, the temperature distribution in the pre-cooling region ofthe DMR process is gradually changed while showing a minimum difference(3K) in the middle of the heat exchange region. In addition, FIGS. 4 and5 show a temperature distribution in a pre-cooling region of aliquefaction process according to the present embodiment andtemperature-entropy lines of ethane and propane cycles in theliquefaction process according to the present embodiment, respectively.

Since the respective pure refrigerants in a pre-cooling step of theliquefaction process according to the present embodiment are introducedin a liquid phase into the heat exchange region (See a reference numeral9 of FIGS. 4 and 5), the temperature of the cold stream has twohorizontal regions (See reference numerals 9-10 and 1112 of FIG. 4),corresponding to vaporization of the ethane refrigerant and the butanerefrigerant. On the other hand, since the butane refrigerant after thenatural gas is pre-cooled is condensed while being subjected to amulti-stage of compressing and cooling processes through the compressorsand the coolers and is then introduced again in a liquid phase into theheat exchange region (See a reference numeral 5 of FIGS. 4 and 5), thetemperature of the hot stream has only one horizontal region (Seereference numeral 6-7 of FIG. 4), corresponding to the condensation ofthe ethane refrigerant.

FIGS. 6 to 8 show exergy utilization and irreversibility in apre-cooling step of the above-mentioned three processes, that is, theC3/MR process, the DMR process, and the liquefaction process accordingto present embodiment, respectively. Exergy efficiencies defined as aratio of an increase in exergy to power input were 34.3%, 30.5%, and31.5% in the respective liquefaction processes as shown in FIGS. 6 to 8,respectively. As described above, when considering that the C3/MRprocess has a disadvantage that a large number of equipments arerequired since a plurality of pressure steps are required and the DRMprocess has a disadvantage that a structure itself is complicated and anoperation of a liquefaction system is difficult since a mixedrefrigerant is used, it may be confirmed that an effect of theliquefaction process according to the present embodiment is excellent.

That is, in the liquefaction process according to the presentembodiment, since the respective pre-cooling cycles pre-cool the naturalgas only with a single pressure step, they may be configured only of arelatively small number of equipments, since the respective pre-coolingcycles use the pure refrigerant, the structure itself thereof is simple,and the operation of the liquefaction system is easy, and since theethane refrigerant cycle and the butane refrigerant cycle are disposedin parallel with each other to pre-cool the natural gas in the same heatexchange region, the efficiency of the liquefaction process is high. Asa result, the liquefaction process according to the present embodimenthas not only all of the advantages of the structure of pre-cooling thenatural gas using the pure refrigerant and the structure of pre-coolingthe natural gas using the mixed refrigerant but also has a highefficiency (the efficiency of the liquefaction process according to thepresent embodiment is very excellent when considering that the C3/MRprocess among the liquefaction processes known up to now is one of theprocesses having very high efficiency).

In addition, the irreversibility in the pre-cooling step of therespective liquefaction processes may be represented by four groups ofvalve V, after-cooler (AC), compressor (C), and heat exchanger (HX) asshown in FIGS. 6 to 8. When comparing the liquefaction process accordingto the present embodiment with the C3/MR process, it may be confirmedthat the irreversibility by the valve is relatively larger in the C3/MRprocess than in the liquefaction process according to the presentembodiment. In addition, when comparing the liquefaction processaccording to the present embodiment with the DMR process, it may beconfirmed that the irreversibility by the after-cooler is relativelylarger in the DMR process than in the liquefaction process according tothe present embodiment. As a result, it may be confirmed that althoughthe irreversibility by the valve and the after-cooler is lower in theliquefaction process according to the present embodiment than in theabove-mentioned two liquefaction process, the irreversibility by theheat exchanger is high in the liquefaction process according to thepresent embodiment than in the above-mentioned two liquefactionprocesses.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims. Accordingly, suchmodifications, additions and substitutions should also be understood tofall within the scope of the present invention. Therefore, the scope andspirit of the present invention should be understood only by thefollowing claims, and all of equivalences and equivalent modificationsto the claims are intended to fall within the scope and spirit of thepresent invention.

INDUSTRIAL APPLICABILITY

The present invention relating to a natural gas liquefaction process inwhich since the respective pre-cooling cycles pre-cool the natural gasonly with a single pressure step, they may be configured only of arelatively small number of equipments, since the respective pre-coolingcycles use the pure refrigerant, the structure itself thereof is simple,and the operation of the liquefaction system is easy, and since twopre-cooling cycles are disposed in parallel with each other to pre-coolthe natural gas in the same heat exchange region, the efficiency of theliquefaction process is excellent has an industrial applicability.

1. A natural gas liquefaction process of pre-cooling natural gas using aclosed loop pre-cooling cycle and liquefying the pre-cooled natural gasusing a closed loop liquefying cycle, wherein the closed looppre-cooling cycle includes first and second pre-cooling cycles forpre-cooling supplied natural gases together in the same first heatexchange region through the respective pure refrigerants, and the closedloop liquefying cycle includes at least one liquefying cycle forliquefying the pre-cooled natural gas through a mixed refrigerant, thefirst and second pre-cooling cycles being a closed circuit coolingcycle.
 2. The natural gas liquefaction process according to claim 1,wherein the pure refrigerant of the first pre-cooling cycle is ethane(C2), and the pure refrigerant of the second pre-cooling cycle is butane(C4).
 3. The natural gas liquefaction process according to claim 1,wherein the first and second pre-cooling cycles include a step ofcompressing the pure refrigerant, a step of cooling the compressedrefrigerant, a step of additionally cooling the cooled refrigerant inthe first heat exchange region, and a step of expanding the additionallycooled refrigerant.
 4. The natural gas liquefaction process according toclaim 1, wherein the closed loop liquefying cycle includes a step ofcompressing the mixed refrigerant, a step of after-cooling thecompressed refrigerant, a step of additionally cooling the cooledrefrigerant in the first heat exchange region to partially condense thecooled refrigerant, a step of separating the partially condensedrefrigerant into a liquid-phase refrigerant portion and a gas-phaserefrigerant portion according to a difference in boiling points, a stepof primarily cooling the pre-cooled natural gas in a second heatexchange region using the liquid-phase refrigerant portion, and a stepof secondarily cooling the primarily cooled natural gas in a thirdheat-exchange region using the gas-phase refrigerant portion.
 5. Thenatural gas liquefaction process according to claim 4, wherein the stepof primarily cooling the pre-cooled natural gas includes a first step ofcooling the liquid-phase refrigerant portion through heat exchange inthe second heat exchange region, a second step of expanding therefrigerant portion cooled in the first step, and a third step ofheat-exchanging the refrigerant portion expanded in the second step andthe natural gas in with each other the second heat exchange region tocool the natural gas.
 6. The natural gas liquefaction process accordingto claim 4, wherein the step of secondarily cooling the primarily coolednatural gas includes a cooling step of cooling the gas-phase refrigerantportion through heat exchange in the second heat exchange region, acondensing step of condensing the refrigerant portion cooled in thecooling step through heat exchange in the third heat exchange region, anexpanding step of expanding the refrigerant portion condensed in thecondensing step, and a step of heat-exchanging the refrigerant portionexpanded in the expanding step and the natural gas with each other inthe third heat exchange region to cool the natural gas.